Unauthorized copying and distribution of motion pictures represents a significant source of lost revenue for the movie studios. Advances in technology have made it easier to make and distribute unauthorized copies of such materials. There exists a need to identify unauthorized copies and to track them to the source of the copy. One method to identify the source of the copy is to expose motion picture print film with a latent image watermark over the entire image forming area of the film before or after printing the motion picture content. The watermark pattern may be digitally created and can be designed such that it is not visually objectionable when viewing the final motion picture. The watermark pattern can be subsequently extracted from an unauthorized copy of the film and used to determine the source of the copy. The watermark can have the visual effect of adding a slight amount of noise or “grain” to the image, which may not be objectionable, but such an exposure also alters the effective sensitometric characteristics of the film, adversely modifying the tone scale of the reproduced image.
In commonly-assigned, co-pending U.S. patent application Ser. No. 10/364,488, Roddy et al, it is shown that exposing film with a watermark preferably requires the modification of the film sensitometric response to compensate for the watermark. This compensation, namely, increasing the contrast in the low density or “toe” portion of the film's sensitometric response, requires that the film's emulsion be reformulated, which is an expensive and time consuming undertaking. While this approach produces the optimal image characteristics, there may be circumstances involving manufacturing costs or schedules in which it may be desirable to apply a watermark to motion picture print film without modifying the film's sensitometric characteristics. However, doing so will significantly degrade the tone scale of the final print, and this degradation cannot be corrected by conventional means in the motion picture production laboratories during printing operations. Some alternative means of compensation must be employed.
The concept of modifying an image digitally is well known in the imaging industry. In patent publication U.S. 2003/0079130 Al, Reed teaches the use of a look-up table to preserve the integrity of a watermark through the dot gain inherent in ink based printing. Look-up tables have been used to compensate for spatial irregularities existing within digital printing devices, as is disclosed in patent publication U.S. 2002/0126199 Al (Yarid et al) and U.S. Pat. No. 6,384,856 B2 (Nakagawa et al). Look-up tables have also been applied to the problem of maintaining tonal consistency between hard-copy (printed) images and soft-copy (e.g., CRT) displays (U.S. Pat. No. 5,671,070 A). Additionally, in U.S. Pat. No. 5,155,588 A, Levien teaches the use of stored gamma correction values on a point basis for selectively modifying a region of an image by manually identifying the region through the use of a display screen and computer trackball or mouse.
However, the problem created by the exposure of the motion picture print film with a watermark is not one of spatial correction. It is a problem of modifying tone scale based upon image content and watermark exposure. Prior art spatial correction approaches for modifying individual images are impractical for modifying the scene content in a full length motion picture, as a typical two hour long movie contains over 172,000 individual images. Therefore, it can be seen that a need exists to provide a means for modifying motion picture images to compensate for the sensitometric effects of a watermark exposure on print film.